The invention relates to a process for the preparation of epoxides by gas-phase oxidation of hydrocarbons having olefinic double bonds by means of a reaction under heterogeneous catalysis, which process can be used in particular in the chemical industry.
It is known that the oxidation of ethene to ethene oxide is carried out industrially virtually exclusively in the gas phase. The industrial catalysts used contain silver as the active component and optionally further promoters and activators, such as, for example, Cs, Ba or K [K. Weissermel and H.-J. Arpe, Industrielle Organische Chemie [Industrial Organic Chemistry], 5th Edition, VCH Verlag, Weinheim, 1998, pages 159-165]. The gas-phase oxidation of propene or other alkenes which have methylene or methyl groups in the xcex1 position leads only to very low epoxide selectivities over these or similar catalysts at an economical alkene conversion. For this reason, propene oxide is currently prepared by an indirect route in the liquid phase, in particular by the chlorohydrin process or by oxidation with hydroperoxides. However, the chlorohydrin process leads to large chlorine losses in the form of useless CaCl2 or NaCl solutions, which pollute the environment. The oxidation with hydroperoxides leads to large amounts of coproducts, such as, for example, styrene or tert-butanol, which adversely affect the economy of the process [Industrielle Organische Chemie (Industrial Organic Chemistry), ibid., pages 291-297; Ullmann""s Encyclopedia of Industrial Chemistry, 5th ed., VCH Verlag, Weinheim, 1993, Vol. A22, pages 23-251; Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed., John Wiley and Sons, New York, 1996, Vol. 20, pages 271-287]. At present, there is no industrial process for the direct oxidation of propene to propene oxide in the gas phase. Olin (USA) operates a pilot plant for the direct oxidation of propene, which is catalyzed by a lithium nitrate melt. In this technically complicated process, relatively large amounts of different byproducts, such as, for example, methanol, acetaldehyde, carbon dioxide and carbon monoxide, are formed in addition to propene oxide [Industrielle Organische Chemie (Industrial Organic Chemistry), ibid., page 300; Ullmann""s Encyclopedia of Industrial Chemistry, ibid., page 251; Kirk-Othmer Encyclopedia of Chemical Technology, ibid., page 288]. The patent WO 98/30552 describes a process for the oxidation of propene over a silver/gold catalyst. A disadvantage of the invention is that a propene oxide selectivity of 50% is possible only at low propene conversions of 2%. A further disadvantage of this invention is the use of large amounts of carbon dioxide as a promoter for the reaction (the recommended concentration in the starting gas is stated as 5-25%). The patents WO 98/00413, WO 98/00414 and WO 98/00415 describe processes for the oxidation of propene in the gas phase over a gold catalyst. A disadvantage of the invention is that only low yields of propene oxide can be achieved. A further disadvantage of the inventions is also that the cost-efficiency of the process is adversely affected by the use of an expensive catalyst.
The aim of the invention is to eliminate the disadvantages described and to develop a technologically simple process which permits the partial oxidation of hydrocarbons having olefinic double bonds under heterogeneous catalysis to give epoxides with relatively high conversions and selectivities, the catalyst used being distinguished by high long-term activity and stability, abrasion resistance and capability of being regenerated.
It is the object of the invention to develop a process which, through the use of an economical solid catalyst and a gaseous oxidizing agent, leads to the epoxidation of hydrocarbons having olefinic double bonds, in particular alkenes, such as, for example, propene, in the gas phase with high epoxide selectivities.
According to the invention, the object is achieved if the solid catalyst is based on an oxide support, in particular silica (SiO2), iron oxide is present as an active component and synergistic elements from Group 1 and/or Group 2 of the Periodic Table of the Elements are present as promoters. The fact that the active component having an iron content of between 0.001% and 1% is uniformly and homogeneously distributed on and/or in the support and the carrier has a specific surface area of more than 50m2/g has been found to be advantageous. Furthermore, it is advantageous if the promoter, in particular sodium ions, is applied by impregnation of the SiO2 matrix, which already contains active component, up to a concentration of 1% and the xe2x80x9cgreenxe2x80x9d catalyst thus prepared is made ready for use by calcination at a temperature between 300 and 1000xc2x0 C. It was found that a pure or an Na ion-promoted oxide support, in particular SiO2, has no or only very little activity and selectivity for the epoxidation of hydrocarbons having olefinic double bonds and, according to the invention, the presence of iron on and/or in the matrix of the oxide support, in particular SiO2, is decisive for the catalytic activity and selectivity in the epoxidation reaction.
When the catalyst thus prepared is used in a flow apparatus, nitrous oxide in concentrations of 5% to 99% is used, according to the invention, for the oxidation. For example, nitrogen, helium or alkane is used as an inert gas. The reaction is carried out in a temperature range of 300-500xc2x0 C. at space velocities of 1-20 |hxe2x88x921gCatxe2x88x921, so that, according to the invention, the epoxide selectivity is between 10 and 70% at conversions of 2-30%. It is advantageous that the catalyst according to the invention loses its activity only to an insignificant extent during the reaction, so that it needs to be reactivated by xe2x80x9cburning offxe2x80x9d in air or in another oxygen-containing gas only after an oxidation time of 5-20 h. Also remarkable is the fact that the catalyst is highly active, mechanically stable and abrasion-resistant even after more than 50 reactivation cycles.
The invention is to be explained below in 6 examples.